Fluid flow systems for internal combustion engines and other liquid utilizing devices frequently include filters for the purpose of removing dirt or other foreign materials from the fluid utilized. To permit continuous operation of the engine or other device supplied by such a filterized system, it is frequently desirable to provide a duplex filter system comprising dual filterized fluid flow circuits to permit maintaining the flow of fluid to the engine while a filter change is being made.
The basic duplex filter system employs a primary circuit and an auxiliary circuit arranged in parallel, with a selector valve arranged to selectively direct fluid flow to either of said circuits. Typically, the selector valve is used to direct fluid flow to one circuit while the filter in the other circuit is changed. It is usual to purge the empty or alternate circuit of air prior to discontinuing flow in the working circuit and redirecting it to the alternate circuit by means of a fill valve placed downstream from the selector valve. This fill valve connects the primary and auxiliary circuits, and when opened, permits fluid flow into the empty circuit, thereby replacing any air present in such circuit.
Other components are typically included in such filter systems, such as check valves to prevent reversal of flow within the fluid circuits, filter by-pass valves, and petcocks or drain valves.
It is generally of great importance, for example, when supplying liquid fuel to an engine, that the alternate circuit be completely purged of air prior to redirecting the flow from the working circuit to prevent cavitation and insure a continuous air-free supply to the engine. Conventional means heretofore employed for determining the sufficiency of charge in the alternate circuit and for determining the presence of air therein have frequently been unsatisfactory and of limited usefulness. Typically, petcocks have been provided on conventional purge tubes for the circuits. Such petcocks have been opened on the alternate circuit at the same time as the fill valve and the tubes observed for fluid discharge. When liquid is discharged from an open tube, the associated circuit is considered sufficiently charged, and the fill valve and petcock are closed. The selector valve is then positioned to redirect fluid flow to this circuit.
There are several disadvantages associated with such a method for determining full fluid charge in the alternate circuit. For instance, the attendant fluid spillage is highly undesirable, particularly in confined spaces such as are present in many marine engine applications. Additionally, the purge tube must be more or less continually observed when the petcock is open, so that any liquid discharge may be terminated as quickly as possible. Further, in such conventional systems, there is no means for detecting the presence of fluid pressure in the circuit having the filter to be replaced. This is of importance, since malfunction of any of several conventional components, such as a check valve, may result in an unexpected retention of fluid pressure in the circuit, and a resultant spillage and/or safety hazard during removal of the filters.